Waste yeast discharged from food production plants such as breweries is subjected to waste treatments by incineration or the like, except for part of the waste yeast being used as a raw material for yeast extracts and yeast preparations, feeds for livestock, fertilizers, and the like. Meanwhile, a portion of yeast cell walls remaining after extraction of yeast extracts is mainly discarded while the other portion of the yeast cell walls is used for health foods, feeds for livestock, and the like.
However, the waste treatments entail costs of transport to disposal sites and disposal costs. Moreover, in the cases where the waste yeast is used for the above-described foods, feeds, fertilizer, and the like, conventional usages have some limitations in terms of the applications, the amount used, and the added values of products, when the amount of the waste yeast generated is taken into consideration. Hence, there are expectations for a treated material which can be used for new applications, can increase the amount used, and has higher added values, and for a treatment method therefor.
Patent Literature 1 describes a method for obtaining an intracellular substance by destroying cells with high-temperature high-pressure steam, and shows yeast as a preferred microorganism. Since the reaction is conducted in a low-temperature region, which is an ionization reaction region, decomposition to low-molecular weight substances hardly proceeds, and the amount of reducing substances produced is very small. Hence, it is difficult to keep the intrinsic potential within a reducing region.
Patent Literature 2 describes a method for obtaining a water-soluble fraction of cell walls of a microorganism by use of high-temperature and high-pressure water at a temperature higher than 100° C. and at a pressure not higher than the saturation vapor pressure at the temperature, and shows yeast as the microorganism. The water-soluble fraction obtained by the method has an oxidation-reduction potential of 0 mV or less immediately after being obtained, but soon the oxidation-reduction potential turns to 0 mV or higher.